1. Field of the Invention
The present invention relates to the design of an integrated circuit, particularly to an oscillator based on a 6T SRAM for measuring the Positive Bias Temperature Instability and the Negative Bias Temperature Instability.
2. Description of the Prior Art
The reliability test of the integrated circuit depends on the reliability of the semiconductor device basically. The reliability is a very important factor to the integrated circuit. As for current nano-device, the reliability plays a very important role to the smaller device and more complicated circuit.
There are three kinds of reliability analysis mainly, including the Time Dependent Dielectric Breakdown (TDDB), the hot-carrier effect of device channel and electromigration effect of metal wire. The hot-carrier effect is the most important conventional reliability analysis. However, as the supply voltage is dropping constantly, the hot-carrier effect is also dropping constantly, therefore the hot carrier has not already been the No. 1 killer of reliability, and the substitute is the Bias Temperature Instability. The Bias Temperature Instability includes the Negative Bias Temperature Instability (NBTI) and the Positive Bias Temperature Instability (PBTI). Among these, the Negative Bias Temperature Instability (NBTI) is caused by the P-type metal-oxide-semiconductor (PMOS) transistor, and the Positive Bias Temperature Instability (PBTI) is caused by the N-type metal-oxide-semiconductor (NMOS) transistor.
The Bias Temperature Instability will cause the variation of critical voltage of transistor. For example, when a negative voltage is applied on the gate, the critical voltage of P-type metal-oxide-semiconductor (PMOS) transistor will be reduced with respect to time. The variation of critical voltage is a great challenge to the operation of integrated circuit. Due to the critical voltage represents the voltage required to open the transistor in the circuit design, the variation represents the uncertain state of transistor and the risk of circuit operation.
It is found by the conventional research that the effect of the Negative Bias Temperature Instability (NBTI) on the P-type metal-oxide-semiconductor (PMOS) transistor is much severer than the effect of the Positive Bias Temperature Instability (PBTI) on the N-type metal-oxide-semiconductor (NMOS) transistor. However, the high-K material and metal gate technique are adopted in 45 nm complementary metal-oxide-semiconductor (CMOS) process. At this time, the importance of the Positive Bias Temperature Instability (PBTI) and the Negative Bias Temperature Instability (NBTI) is the same, but the effect and mechanism of the Positive Bias Temperature Instability (PBTI) and the Negative Bias Temperature Instability (NBTI) are different. Thus, an oscillator based on a 6T SRAM is required to measure the Positive Bias Temperature Instability (PBTI) and the Negative Bias Temperature Instability (NBTI), in order to help the circuit designers to realize the dynamic and real-time change of long-term reliability.